FIG. 1 is a plan view of a conventional InGaAs planar PD described for example in Collection of Drafts of Lectures at 1986 General National Meeting of The Institute of Electronics and Communication Engineers of Japan, No. 978, pages 4-149. FIG. 2 shows a cross-sectional view of the device shown in FIG. 1 along the line 2--2. In FIG. 2, on an n.sup.+ -type InP substrate 1, an n-type InP buffer layer 2, an n.sup.- -type InGaAs light absorbing layer 3, and an n.sup.- -type InP window layer 4 are disposed in the named order in a stack. A p-type impurity, such as Zn, is diffused through part of the surface of the n.sup.- -type window layer 4 to form a reversed conductivity type region, namely, a p.sup.+ -type region 5. The bottom portion of the p.sup.+ -type region 5 is in contact with the n.sup.- -type InGaAs light absorbing layer 3. A broken line 6 represents a front of a depletion layer 31. The depletion layer front 6 serves substantially as a PN junction between the p.sup.+ -type region 5 and the n.sup.- -type InP window layer 4 and the n.sup.- -type InGaAs light absorbing layer 3.
Over the top surface of the n.sup.- -type InP window layer 4, except for a light receiving region 10, a light-transparent surface protection film 11 of, for example silicon nitride (SiN) is deposited by, for example, plasma CVD. A p-electrode (anode) 12 is disposed on the surface protection film 11 along the periphery of the light receiving region 10. The p-electrode 12 is in ohmic contact with the p.sup.+ -type region 5. As shown in FIG. 1, a portion of the p-electrode 12 extends outward to provide an electrode pad 13 for wire bonding. An anode wire 14 is bonded to the electrode pad 13. Returning again to FIG. 2 an n-electrode (cathode) 15 is disposed on the bottom surface of the substrate 1. Incident light 16 generates pairs of electrons 17 and holes 18 in the light absorbing layer 3.
Now the operation of the conventional photodiode shown in FIGS. 1 and 2 is explained. The InGaAs light absorbing layer 3 which is formed by crystal growth on the n.sup.+ -type InP substrate 1 and has a lattice constant matching that of the InP substrate 1 has a bandgap wavelength .lambda.g of about 1.67 .mu.m. The bandgap wavelength .lambda.g of the InP window layer 4 is about 0.93 .mu.m. Accordingly, the range of wavelengths of light to which the InGaAs photodiode shown in FIGS. 1 and 2 is sensitive is approximately from 1.0 .mu.m to 1.6 .mu.m (.lambda..perspectiveto.1.0-1.6 .mu.m). Therefore, in the following explanation, the wavelength .lambda. of incident light 16 is assumed to be about 1.3 .mu.m.
Generally a photodiode is used with no bias or with a reverse bias applied to it. In the conventional photodiode shown in FIGS. 1 and 2 too, 0 V or a negative voltage of from -5 V to -10 V with reference to the potential at the n-electrode 15 is applied to the p-electrode 12. The carrier concentration of the n.sup.- -type InP window layer 4 is on the order of 1.times.10.sup.16 cm.sup.-3 and that of the n.sup.- -type InGaAs light absorbing layer 3 is on the order of 5.times.10.sup.15 cm.sup.-3 which is smaller than that of the window layer 4. Accordingly, under the reverse bias condition in particular, the depletion layer 31 extends chiefly in the light absorbing layer 3.
When light 16 having .lambda..perspectiveto.1.3 .mu.m is incident on the light receiving surface, it is not absorbed by the n.sup.- -type InP window layer 4 having .lambda.g.perspectiveto.0.93 .mu.m but is absorbed by the n.sup.- -type InGaAs light absorbing layer 3 of which .lambda.g is about 1.67 .mu.m. This causes the generation of pairs of electrons 17 and holes 18 in the light absorbing layer 3. The carriers comprising pairs of electrons 17 and holes 18 generated in the depletion layer 31 within the layer, 3 are extracted through the electrodes 12 and 15, as a drift current caused by the electric field in the depletion layer 31, which current is monitored as photocurrent by an external circuit. Those holes which are generated outside and diffuse into the depletion layer 31 are extracted as part of the drift current. The magnitude of the photocurrent and hence the sensitivity of the device is increased by lowering the carrier concentration in the n.sup.- -type InGaAs light absorbing layer 3, which increases the width of the depletion layer 31 so that the proportion of carriers contributing to the drift current increases. This also increases the breakdown voltage. Furthermore, by choosing the values of the thickness t and the index of refraction n of the surface protection film 11 in relation to the index of refraction n.sub.s of the n.sup.- -type InP window layer 4, the index refraction in vacuum, n.sub.o, and the wavelength .lambda. of incident light 16 such that the following expressions (1) and (2) are fulfilled the surface protection film 11 has a zero reflectivity or 100% transmissivity, that is, the surface protection film 11 becomes a so-called AR coating film. ##EQU1##
The sensitivity of the photodetector device can be increased by about 40% by using an AR surface protection film.
In the conventional InGaAs planar-type photodiode with the above-described structure, light 16 is incident on the entire surface of the photodiode, so light incident on areas other than the light receiving region 10 and advancing into regions other than the depletion layer 31 also causes pairs of electrons 17 and holes 18 to be generated. Holes which diffuse into the depletion layer 31 are extracted as photocurrent. However, because the diffusion speed is significantly lower than the drift speed, a delay is exhibited between the photocurrent based on the carriers generated within or near the depletion layer 31 and the photocurrent based on the holes generated outside and diffusing into the depletion layer 31 when they are monitored by an external circuit. FIG. 3 shows the example of a waveform of pulse-shaped incident light 16, and FIG. 4 shows photocurrent generated in response to the intensity variations of the incident light 16 shown in FIG. 3. The incident light 16 has a pulse-shaped waveform, as shown in FIG. 3, which rises at a time t.sub.1, starts to fall at a time t.sub.2, and reaches a minimum a time period t.sub.f1 after t.sub.2. The photocurrent monitored by the external circuit will have a waveform 20 shown in FIG. 4, which falls to a minimum a time period t.sub.f2 which is longer than t.sub.1, after the time t.sub.2. From this, it is understood that the above-described conventional InGaAs planar-type photodiode cannot respond fast to incident light which changes at a high speed.
Now, several examples of known photodiodes having an improved photocurrent response characteristic are described.
(1) Photodiode shown in FIG. 1 of Japanese Unexamined Patent Publication No. HEI 1-161778:
The photodiode shown in FIG. 1 of Japanese Unexamined Patent Publication No. HEI 1-161778 is formed by removing those portions of a window layer and a light absorbing layer which are outside a PN junction near a p.sup.+ -type diffusion region. Since no carriers are generated by light absorption outside the junction formed by the p.sup.+ -type region, no photocurrent which would otherwise result from holes diffused into a depletion region is present. Thus, the photodiode exhibits an improved photocurrent-to-incident light response. However, in order to produce this photodiode, it is necessary to completely etch away the portions of the window layer and the light receiving layer outside the PN junction which is disadvantageously time consuming. Another disadvantage is that portions of a protection film and conductor leads disposed over steps formed by such an etching process are susceptible to defects and disconnections depending on the step coverage.
(2) Semiconductor photodetector device shown in FIG. 1 of Japanese Unexamined Patent Publication No. HEI 2-214171:
This photodetector device includes a ring-shaped floating p.sup.+ -type region which surrounds a light-receiving p.sup.+ -type diffusion region. The field formed within the depletion layer generated by the floating p.sup.+ -type region prevents the diffusion of carriers which could cause a slow response. However, it is difficult to control the diffusion of an impurity, such as Zn, so as to provide the desired depth and thickness for the floating p.sup.+ -type region, and consequently, the prevention of the diffusion of carriers which could cause slow response is insufficient.
(3) Semiconductor photodetector device shown in FIG. 1 of Japanese Unexamined Patent Publication No. HEI 2-240974:
The semiconductor photodetector device disclosed in this publication comprises a stack of an HR (highly reflective) coating, a light absorbing region, and an AR (anti-reflective) coating in the named order along the periphery of a light receiving region, whereby light is prevented from entering into the crystal through portions other than the light receiving region. Due to difficulty in forming the structure surrounding the light receiving region, in particular, difficulty in forming the crystalline light absorbing region on the HR coating, it is relatively difficult to fabricate this photodetector device.
(4) Semiconductor photodetector device shown in FIG. 1 of Japanese Unexamined Patent Publication No. HEI 1-310579:
The photodetector device disclosed in this publication includes an n.sup.+ -type InP substrate with a groove formed therein, an InGaAs light absorbing layer disposed in the groove, and p.sup.+ -type diffusion region disposed on the light absorbing layer and on the surface portion of the substrate around the light absorbing layer. The crystallographic properties of the light absorbing layer disposed within the groove are not good, and the depletion layer extends across the entire light absorbing layer so the proportion of current attributable the light entering through regions other than &he light receiving region is large.
(5) Photodiode shown in FIG. 1 of Japanese Unexamined Patent Publication No. HEI 2-291180:
A high concentration region is disposed around a p.sup.+ -type region in a substrate. A maximum field intensity at the PN junction between the substrate and the p.sup.+ -type region increases in the high concentration region, which undesirably reduces the withstanding voltage of the device and increases surface leakage current.
(6) Photodiode shown in FIG. 1 of Japanese Unexamined Patent Publication No. HEI 1-205477:
The photodiode disclosed in this publication includes a high impurity concentration region of p.sup.+ -type conductivity which is opposite to that of a light receiving layer and a window layer, disposed around a light receiving portion. The p.sup.+ -type high impurity concentration region is not for the purpose of preventing diffusion current, which could cause a slow response speed, from being monitored in an external circuit, but for the purpose of suppressing surface leakage current. Accordingly, this p.sup.+ -type high impurity concentration region does not improve the speed of response to incident light changes.